Our long-term goal is to understand how neurons acquire their individual identities and how they express those identities by developing the appropriate set of differentiated characteristics, including cell shape and axonal projection. Appropriate expression of these characteristics is important for proper nervous system function. We plan to study these issues in embryonic zebrafish where we can examine the development of individually identified motoneurons in living embryos. We hypothesize that individual neurons have intrinsic differences that arise, at least in part, because of environmental interactions such as: i) inductions that determine which cells adopt a motoneuronal fate, ii) position-dependent signals that determine axonal trajectories and thus synaptic targets of individual motoneurons and iii) competitive interactions that determine the fates of motoneurons that form an equivalence pair. We propose to examine the nature of morphological differences between individually identified motoneurons. By culturing these neurons singly, we will learn whether these differences are intrinsic to the cells, or require extrinsic environmental signals. By culturing neurons at different times, we will also learn when these differences arise. We propose to examine whether inductive signaling from segmented mesoderm patterns motoneurons in the spinal cord. We will test this idea by creating genetic mosaics by transplanting precursors of segmented mesoderm between wild types and mutants which lack segmented mesoderm and motoneurons. We propose to learn about the genetic control of motoneuronal phenotype. We will produce and isolate mutants in which motoneuronal development is altered. We will characterize these mutants to learn about the specific defects. We will be particularly interested in mutants in which position- dependent signals are affected. We propose to learn about competitive interactions between specific motoneurons that form an equivalence pair. We will transplant individual motoneurons to new locations to examine the interactions between them. We will learn whether death of a specific motoneuron can be prevented, and we will learn its fate when it survives. We will learn whether motoneurons that do not typically behave as an equivalence pair can be induced to do so.